1. Field of the Invention
The present invention relates to a nanostructure comprising nanowires vertical or almost vertical to a film face, and a method for production of the same, and particularly to a nanostructure comprising nanowires in which narrow wires having very small diameters are integrated at a high density, characterized in that the average diameter of the nanowires is 20 nm or less, and the length of the nanowire is 1 μm or less. The nanostructure can be used in a wide range as functional materials such as electronic devices and micro-devices, structural materials and the like. Particularly, it can be applied as quantum effect devices, light emitting devices, solar batteries, catalysts, electrochemical sensors, biosensors and the like.
2. Related Background Art
Thin films, narrow wires and dots of metals and semiconductors may exhibit specific electric, optical and chemical natures due to confinement of movement of electrons in a size smaller than a certain characteristic length. Particularly, since the carbon nanotube was discovered, not only semiconductor quantum dots and quantum narrow wires of Si, Germanium (Ge) and the like but also metal nanostructures of Ag, Cu and the like have received attention.
Nanowires and nanodots can be used in electrochemical sensors and catalyst materials by taking advantage of their specific structures and characteristic properties such as electric conductivities and heat conductivities. The catalytic activity depends on factors such as the metal particle size, the particle density, the particle form, the surface atomic arrangement and the electronic structure. Nanowires having a large surface area and a characteristic structure have the potential for becoming a catalyst having a high activity. In addition, the nanowire can be used as a high efficiency electrode by taking advantage of its large surface.
As a method for production of nanowires, a fabrication method using a carbon tube and silica as a mold on a film, and the like can be considered. However, in these methods, it is difficult to produce nanowires having very small diameters and integrated at a high density.
Other methods for fabrication of nanowires include methods using microprocessing techniques. The microprocessing techniques include, for example, semiconductor processing techniques including fine pattern drawing techniques such as electron beam exposure and X-ray exposure as well as photolithography. Fabrication of nanostructures by such semiconductor processing techniques is poor in yield and increases the equipment cost, and thus a method enabling nanostructures to be fabricated easily and with good reproducibility is desired.
In addition to the fabrication methods by semiconductor processing techniques, there is an attempt to realize a new nanostructure using as a base a naturally formed regular structure, i.e. a structure formed in a self-organizing manner. This method has, among other things, the following advantages: 1) there is a possibility that a fine and specific structure better than those by conventional methods can be fabricated depending on the microstructure used as a base; and 2) a nanostructure having a large area can be generally fabricated.
Examples of specific structures formed in a self-organizing manner include alumina (Al) anodic oxidation coatings (RC. Furneaux, W. R. Rigby, and A. P. Davidson, Nature, Vol. 337, p147, 1989). When an Al film formed on substrate or an Al plate is anodized in an acidic electrolyte, porous oxidation coatings (anodized alumina) are formed (see RC. Furneaux, W. R. Rigby, and A. P. Davidson, Nature, Vol. 337, p147, 1989).
The feature of the porous oxidation coatings is that it has a specific geometrical structure in which very small columnar pores (nanoholes) having diameters of several nm to several hundreds nm are arranged in parallel with the space of several tens nm to several hundreds nm (cell size). The columnar pore has a high aspect ratio and is relatively excellent in evenness of diameters of sections if the space between pores is several tens nm or greater. The diameter and space of the pore can be controlled to some extent by adjusting the type of acid and the voltage during anodic oxidation.
Methods for fabrication of a nanostructure having nanosize pores include anodization of Si in addition to the anodic oxidation alumina described above. The anodization of Si is a method of forming porous Si by anodizing crystal Si or polycrystal Si in an aqueous solution having hydrofluoric acid (HF) as a base (Masuda, Kotai-Butsuri (Solid State Physics), Vol. 31, p493, 1996).
The porous Si has on the surface or in the interior thereof an infinite number of micropores each having a size of 1 nm to several tens nm under specific fabrication conditions. If seen macroscopically, the micropore is almost vertical to the film face, but its shape and density significantly vary depending on the conditions of anodization.
Applications of anodic oxidation structures fabricated in a self-organizing manner are explained in detail by Masuda (see Masuda, Kotai-Butsuri (Solid State Physics), Vol. 31, p493, 1996). They include, for example, an application as coatings utilizing abrasion resistance and insulation resistance of anodic oxidation coatings, and an application of peeling away the coatings to form a filter. Moreover, they are tried for various applications including coloring, magnetic recording media, EL light emitting devices, electrochromic devices, optics, solar batteries and gas sensors by using techniques of filling metals, semiconductors and the like in nanoholes, and techniques of replicas of nanoholes.
From this point of view, the self-regulating or self-forming method, particularly the method of Al anodic oxidation or Si anodization is capable of fabricating the nanostructure easily and in a large area, and is most suitable as a method for forming nanoholes having very small pore diameters at a high density.
In view of this technical background, the inventors conducted studies over and over again, and consequently found a method capable of forming a nanoporous member in which the average diameter of pores is 20 nm or less, and a wall material separating the pores is composed of a semiconductor material (Si, Ge, Si and Ge). It was found that by filling this nanoporous member with various kinds of materials (metal/alloy/semiconductor) and selectively removing only the wall material using a difference in solubility between the fillers and the wall material, a nanostructure comprising nanowires having very small diameters and integrated at a high density could be formed.
The present invention was made with this conventional situation as a background, and provides a nanostructure comprising nanowires having very small diameters and integrated at a high density, and capable of being applied to high-functional devices.